ENHANCED: Support pty top-level communication instead of TCP

ENHANCED: Support pty top-level communication instead of TCP

Add the ability for top-level buffers to communicate with their
backing threads via pty instead of a local TCP connection.  This is
controlled by a new user option, enabled by default on systems where
Emacs can creates a pty.

* sweep.pl (sweep_top_level_start_pty/2): New predicate.
* sweeprolog.el (sweeprolog-top-level-use-pty): New user option.
(sweeprolog-top-level-buffer): Use it.
* sweep.texi (The Prolog Top-level): Document it.